GENEVA (Reuters) - Healthy patients who get H1N1 swine flu without suffering complications do not need to be treated with antivirals like Tamiflu, the World Health Organisation (WHO) said on Friday.

But the drugs are strongly recommended for pregnant women, patients with underlying medical conditions and children under 5, since they are at increased risk of more severe illness.

However, judging who will need antivirals is not easy for doctors since the risk of developing severe illness from the new flu strain is not restricted to people who have chronic conditions such as heart disease, diabetes or asthma.

"Worldwide, around 40 percent of severe cases are now occurring in previously healthy children and adults, usually under the age of 50," the U.N. agency said in its latest guidelines on drug use.

"Some of these patients experience a sudden and very rapid deterioration in their clinical condition, usually on day 5 or 6 following the onset of symptoms," it said.

Despite the WHO's conclusion that underlying medical conditions do not predict a severe infection, the fact that 6 out of 10 patients suffering serious bouts of swine flu do have previous conditions chimes with other medical research.

A study published on Thursday by France's Institute for Public Health Surveillance concluded that about half the people dying from swine flu were pregnant or had other health conditions, especially diabetes or conditions linked with obesity.

The WHO said doctors should give Tamiflu to patients with severe illness or whose condition deteriorates, or to high-risk groups including pregnant women, but not to healthy people with no complications, as most of these recover fully within a week.

International experts who reviewed the latest evidence found that Tamiflu, made by Switzerland's Roche, "can significantly reduce the risk of pneumonia", a leading cause of death for both pandemic and seasonal flu, it said.

Viral pneumonia, which does not respond to antibiotics, can lead to the failure of multiple organs, including the heart, kidneys and liver, so patients will require intensive care and additional treatment to antivirals.

"Studies show that early treatment, preferably within 48 hours after symptom onset, is strongly associated with better clinical outcome," the WHO said.

The recommendation applies to all patients, including pregnant women and all age groups including infants.

When oseltamivir, the generic name for Tamiflu, is not available or cannot be used for any reason, zanamivir, made by GlaxoSmithKline under licence from Biota and sold under the brand name Relenza, may be given, it said.

The WHO said it has been notified of 12 cases where the H1N1 virus had been resistant to treatment with Tamiflu, as a result of a mutation. There was no evidence of onward transmission of the virus in these cases, it said.

The H1N1 strain has now spread to 177 countries, causing at least 1,799 deaths, the WHO said in a separate statement.